tests/docker: use --arch-only for installing deps

tests/docker: use --arch-only for installing deps

The Debian QEMU packages require a bunch of cross compilers for
building firmware which aren't available on all host architectures.
Using --arch-only skips this particular requirement and allows us to
install just the dependencies we need.

Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>